The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. (February 1994)], incorporated herein by reference. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
Flexible tape circuits (also called "flex circuits" for short) are particularly suited for connecting electrical components where one or both connections may be coupled to moving parts. For example, in a hard copy apparatus using an ink-jet pen, the pen, connected to a system controller, is scanned across the print media while "shooting" droplets of ink in a dot matrix pattern to form text or images. Flex circuits are also useful where electrical interconnections to microcomponents associated with one device, may be located on different sides of a polyhedral structure making up that device, such as on an ink-jet pen where the electrical connections are on one face and a printhead is on an adjoining face.
In general, flex circuits are fine, conductive filaments or formed traces laminated between, overmolded with, or otherwise adhered to, a layer of a flexible, dielectric material, such as a polyimide tape. The interconnect circuit so formed can be bent or looped without affecting the electrical interconnections between the electrical components it connects. Flex circuits are fabricated individually in accordance with the specific circuit design; for example, a 24 mm wide circuit would be processed on a wider polyimide tape having sprocket holes along one or both edges of the film for use in the manufacturing process.
A "two-layer" flex circuit with metallization exposed on both sides is shown in FIG. 1 (Prior Art). Generally, a tape layer 101 is provided with vias 103, 103' for allowing electrical terminals, such as electrical connector probe tips, or connector pins, 105, 105', to be pressed therethrough and into contact with a metal layer trace, or a circuit contact pad, 107, 107' of the flex circuit. In an ink-jet pen use, the flex circuit is generally adhered (glued, heat staked, or the like) to the pen body (not shown) and the traces of the metal layer are connected to the active electronic components of the pen (such as resistors or piezoelectric transducers used as the ink droplet generators). The probes would generally be part of an electrical connector on a pen carriage, or the like, into which the pen is inserted. In such a structure, the vias 103, 103' must be sized and shaped to ensure a proper contact between the probes 105, 105' and the circuit contact pads 107, 107', respectively.
A "three-layer" flex circuit, also suitable for mounting to an ink-jet pen body, is shown in FIG. 2 (Prior Art). An adhesive layer 201 is used to couple a metallic foil to the tape layer 101. The foil can then be etched, or otherwise processed, to form appropriate circuit contact pads 107, 107'. Note that vias 203, 203' must be punched through both the tape layer 101 and the adhesive layer 201 for the probes 105, 105' to reach the circuit contact pads 107, 107'. Moreover, the vias must be wider to ensure proper contact, allowing for manufacturing tolerances. Thus, three layer adhesive-based flex circuits have been avoided for ink-jet pen use because the effective size of the pen-printer interconnect pads is a function of the centerline-to-centerline distance between the mechanical punch and the amount of the tape and adhesive substrate needed between the vias.
The two-layer flex circuit vias are chemically etched, resulting in effectively larger pads for the same circuit layout, its use has been found to be more expensive.
Therefore, there is a need for flex circuits that can be massed produced, and are of a more reliable and higher functionality, having effective test pad size independent of the more costly two layer flex circuit with its chemically etched vias.